Often going by the common name "gobies", the
Dragonets actually occupy an adjacent Suborder (the Callionymoidei).
"True" Gobioids vary from them in a few obvious ways:
placement and shape of pelvic fins (anterior to the pectorals in
Callionymids, posterior and suction-like in Gobioids), the opening of
the gills (small apertures up behind the head in Callionymids, larger
in gobiods) .The fourteen genera and about 186 species of the family
Callionymidae are typified by small gill openings, by having broad,
depressed heads, and scaleless bodies with two dorsal fins... Living on
the bottom with a characteristic "scooting" type of
locomotion.

The predominantly offered genus is Synchiropus.
Synchiropussplendidus is the Blue, and Synchiropus
picturatus, the Green or Spotted Mandarin. These two have one of
the most dismal survival records of captive marines. Almost all perish
within a month of wild capture... most often due to simple
starvation.

Selection:

The principal selection criteria for picking out healthy
dragonets are their fullness of body and activity level. Suitable
specimens should not be skinny, and should be out and about,
investigating their environment. For sure you want to see the
specimen/s eat.

Selection: (Andy Bulgin
input/corr.) The principal selection criteria for picking
out healthy dragonets are their fullness of body and activity
level. Suitable specimens should not be skinny, and should be out
and about, investigating their environment. For sure you want to
see the specimen/s eat. [If the specimen/s is not actively hunting
for food and/or is skinny, do not buy it.]

Above, an okay "index of fitness" Green/Spotted
Mandarin and one that is too characteristically thin (note,
"line" along body midline).

Habitat:

Some species of mandarins offered in the trade are found
on nothing other than sandy bottoms, but the Green/Spotted and
Blue/Psychedelic species are almost always located in and amongst
(mainly Acroporid) coral rubble, which they only venture out past
sunset to feed and interact with their own kind.

Foods/Feeding:

As stated over and over, feeding, or rather a lack
therein is THE common cause of loss of these animals. They spend many
hours seeking out small live invertebrates living in/on live rock and
substrates. If these are not present or otherwise supplied, you will
see your mandarins sides sink in and its vigor wane. Live foods can be
bought on a regular basis, cultured in separate vessels, in an attached
fishless refugium. Starter cultures for these organisms can be
purchased from companies that you can find on the Internet using the
search terms: "live plankton fish food".

Do not fall into the trap of offering nothing but adult
live brine shrimp, suffused with supplements (e.g. Selcon) or not.
It's not unheard of that a dragonet will accept (with training)
frozen/defrosted foods in place of live, but it is rare that non-live
foods sustain them.

Disease:

Dragonets are notably slimy fishes that are not as
susceptible to external parasite infestations as other fishes. However,
they are not immune, and are overly sensitive to copper compounds,
other metal-based and formalin containing medications. They are best
treated through environmental manipulation (hyposalinity, elevated
temperature) should they show signs of such afflictions.

Disease: (Andy Bulgin input/corr.) Dragonets are
notably slimy fishes that are not as susceptible to external
parasite infestations as other fishes. [For this reason, and
because the typical quarantine setup lacks the live food necessary
to sustain these animals, the quarantine of visibly healthy
specimens is best skipped altogether in favor of an extended pH
adjusted freshwater dip/bath.] However, they are not immune to
disease/infection, and are overly sensitive to copper compounds,
other metal-based and formalin containing medications. They are
best treated through environmental manipulation (hyposalinity,
elevated temperature) should they show signs of such afflictions.
Andy

Foods/Feeding/Nutrition:

Whatever other writers have stated, Mandarins almost
never accept enough of anything other than live foods that are
omnipresent in their system to sustain themselves. A nutrient rich live
rock reef tank, read that as one heavily populated with hard
substrates, with substantial interstitial crustacean and worm, and
other small sessile invertebrate life of about 100 gallons will support
one individual. And this assumes you have no
similar food-competing tankmates.

In the wild their food choices are principally small
crustaceans and worms. You can culture these "incidentally"
in a large main/display system with lots of substrate and/or live rock,
but adding a live fishless refugium will go an immense distance in
assuring your mandarin/s receive sufficient live food. These fishes
cannot live on dried-prepared or frozen/defrosted, or chopped meaty
foods.

Compatibility

Callionymids are very docile when it comes to competing
for food or space and must need be kept with other very easygoing fish
species or perish from harassment or lack of food. Also overly
aggressive invertebrates should not be mixed with them. Dragonets are
good reef aquarium specimens, leaving alone all desirable species, but
may in turn be consumed by anemones, the large coral anemone
(Amplexidiscus), or large crustaceans. Ideal tankmates include
tube-mouthed fishes (seahorses, Pipefishes), small blennies, Jawfishes
and gobies, Dartfishes, flasher and fairy wrasses. Larger wrasses,
Dottybacks, goatfishes, most butterflyfishes, angels and puffers,
triggers are definitely out.

Though seemingly defenseless in their slow swimming and
"scooting" locomotion, this group of fishes is widely
unpalatable… some evidence exists that their body slime is toxic
or at least unpalatable. This and a prominent gill cover spine serves
these fishes well as predator deterrents in most cases. A related note
re this opercular spine; take care to scoop out mandarins with a bag or
specimen container rather than a mesh net, as these can get fouled
easily with the protuberance.

You may well see them kept in a group at a dealers but be
warned, this is not a natural setting, and all species should be kept
either one to a tank or as a single male with more than one female or a
heterosexual pair. Otherwise, unless the tank is very large (hundreds
of gallons) eventually you will see them fighting vigorously.

Reproduction:

For species known, Callionymids are pelagic spawners that
come out past sunset, "do a formalized dance" and release
their gametes into the environment, where they float up into the moving
upper water column via currents. Spawning occurs at about weekly
intervals (Algosaibi 1983), eggs are released of a few hundred in
number, approximately a half to one millimeter in diameter, that hatch
out in about a day. Depending on species, young remain in the plankton
from a week to forty days before settling down. Unfortunately no young
have been raised to date.

Though far less frequently imported, males are easily
distinguished from females by much longer (about three times the
length) first dorsal fins, larger size and brighter coloration.

Some of the Few Dragonets Seen in the
Trade:

Most dragonets are rather bland in coloration, a
smattering of brown, black, yellows… but a few are spectacular
"paisley" prints. All are comical in their behavior.

So, let's review. To successfully keep dragonets one needs a
very peaceful, large reef system with lots of live rock and deep sand
bed and/or such a system with a vibrant fishless refugium (highly
recommended). Don't have this sort of set-up? Leave these fishes in
the ocean.